Microlithographic projection systems project patterns onto substrates for selectively exposing photosensitive layers at multiple stages during the manufacture of microcircuits and microdevices. Often, the patterns must be stitched together to expose extended areas or registered with underlying patterns to build the desired circuits or devices. Image magnification of the projected patterns must be finely controlled to compensate for variations in operating conditions, such as changes in ambient temperature or pressure, to properly relate the patterns in successive exposures. Stepped lithography, which requires the stitching together of adjacent patterns, generally requires magnification control in two orthogonal directions within the image plane of the projection system. Scanning lithography, which regulates exposure times in the scanning direction, generally requires magnification control in only one direction that is orthogonal to the scan direction.
Magnification control is typically administered by relatively translating either (a) an object conjugate (e.g., reticle) to vary a ratio of object to image distances or (b) an object field lens to relatively vary a ratio of corresponding focal distances. Both approaches require the projector systems to be non-telecentric in object space so that the component shifts change magnification. However, the same projection systems are required to be telecentric in image space so that small shifts in focus to not affect the magnification of the projected image. Illumination and projection systems combined under conditions of partial coherence further complicate the angular distribution of rays in object space required to maintain telecentricity in the image space. As a result, the component shifts within object space can, in addition to changing magnification, also distort the images projected into image space or produce wavefront aberrations. A combination of counteracting component shifts can be required to make magnification corrections with a minimum of distortion. Projection systems with telecentric object space have used corrective optics with specially shaped surfaces, variations in air pressure between lens components, and variations in beam frequency for adjusting combinations of magnification and distortion.